Organic electroluminescent device with a high resistant inorganic electron injecting layer

ABSTRACT

To realize an organic EL device having an excellent electron injection efficiency, improved luminous efficiency, low operating voltage, and low cost, the invention provides an organic EL device comprising a hole injecting electrode, an electron injecting electrode, and at least one organic layer between the electrodes, at least one of the organic layer having a light emitting function. The device further has a high resistance inorganic electron injecting layer between the electron injecting electrode and the organic layer. The high resistance inorganic electron injecting layer contains as a first component an oxide of at least one element selected from alkali metal elements, alkaline earth metal elements and lanthanide elements, having a band gap of up to 4 eV and as a second component at least one element selected from Ru, V, Zn, Sm, and In and is capable of blocking holes and has conduction paths for carrying electrons.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to an organic electroluminescent (EL) device andmore particularly, to an inorganic/organic junction structure suitablefor use in a device of the type wherein an electric field is applied toa thin film of an organic compound to emit light.

2. Background Art

Research and development efforts have been made on organic EL devicesfor display applications because the devices can be formed on glass overa substantial area. In general, organic EL devices have a basicconfiguration including a glass substrate, a transparent electrode ofITO etc., a hole transporting layer of an organic amine compound, anorganic light emitting layer of a material exhibiting electronicconductivity and intense light emission such as Alq3, and an electrodeof a low work function metal such as MgAg, wherein the layers arestacked on the substrate in the described order.

The device configurations which have been reported thus far have one ormore organic compound layers interposed between a hole injectingelectrode and an electron injecting electrode. The organic compoundlayers are typically of two- or three-layer structure.

Included in the two-layer structure are a structure having a holetransporting layer and a light emitting layer formed between the holeinjecting electrode and the electron injecting electrode and anotherstructure having a light emitting layer and an electron transportinglayer formed between the hole injecting electrode and the electroninjecting electrode. Included in the three-layer structure is astructure having a hole transporting layer, a light emitting layer, andan electron transporting layer formed between the hole injectingelectrode and the electron injecting electrode. Also known is aone-layer structure wherein a single layer playing all the roles isformed from a polymer or a mixed system.

FIGS. 3 and 4 illustrate typical configurations of organic EL devices.

In FIG. 3, a hole transporting layer 14 and a light emitting layer 15 oforganic compounds are formed between a hole injecting electrode 12 andan electron injecting electrode 13 on a substrate 11. In thisconfiguration, the light emitting layer 15 also serves as an electrontransporting layer.

In FIG. 4, a hole transporting layer 14, a light emitting layer 15, andan electron transporting layer 16 of organic compounds are formedbetween a hole injecting electrode 12 and an electron injectingelectrode 13 on a substrate 11.

Attempts have been made to improve the luminous efficiency of theseorganic EL devices. With the prior art device configuration, however,for reasons of poor electron injection efficiency of the electroninjecting and transporting layer, it was difficult to achieve effectiverecombination in the light emitting layer and hence, to provide a devicewith a fully satisfactory efficiency.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an organic EL devicehaving an excellent electron injection efficiency, improved luminousefficiency, low operating voltage, and low cost.

This and other objects are achieved by the present invention which isdefined below.

1. An organic electroluminescent device comprising

a hole injecting electrode,

an electron injecting electrode,

at least one organic layer between the electrodes, at least one layer ofsaid organic layer having a light emitting function, and

a high resistance inorganic electron injecting layer between saidelectron injecting electrode and said light emitting layer, said highresistance inorganic electron injecting layer comprising as a firstcomponent an oxide of at least one element selected from alkali metalelements, alkaline earth metal elements and lanthanide elements, havinga work function of up to 4 eV and as a second component at least onemetal having a work function of 3 to 5 eV, and said high resistanceinorganic electron injecting layer being capable of blocking holes andhaving conduction paths for carrying electrons.

2. The organic electroluminescent device of (1) wherein said secondcomponent is at least one metal selected from the group consisting ofZn, Sn, V, Ru, Sm, and In.

3. The organic electroluminescent device of (1) wherein said alkalimetal elements include Li, Na, K, Rb, Cs, and Fr, said alkaline earthmetal elements include Mg, Ca, and Sr, and said lanthanide elementinclude La and Ce.

4. The organic electroluminescent device of (1) wherein said highresistance inorganic electron injecting layer has a resistivity of 1 to1×10¹¹ Ω-cm.

5. The organic electroluminescent device of (1) wherein said highresistance inorganic electron injecting layer contains 0.2 to 40 mol %based on the entire components of the second component.

6. The organic electroluminescent device of (1) wherein said highresistance inorganic electron injecting layer has a thickness of 0.3 to30 nm.

7. The organic electroluminescent device of (1) further comprising ahigh resistance inorganic hole injecting layer between said holeinjecting electrode and said organic layer, said high resistanceinorganic hole injecting layer being capable of blocking electrons andhaving conduction paths for carrying holes.

8. The organic electroluminescent device of (7) wherein said highresistance inorganic hole injecting layer has a resistivity of 1 to1×10¹¹ Ω-cm.

9. The organic electroluminescent device of (7) wherein said highresistance inorganic hole injecting layer contains an insulative metalor metalloid and at least one member selected from the group consistingof oxides, carbides, nitrides, silicides and borides of metals.

10. The organic electroluminescent device of (7) wherein said highresistance inorganic hole injecting layer contains

silicon oxide or germanium oxide or a mixture of silicon oxide andgermanium oxide as a main component, the main component beingrepresented by the formula: (Si_(1-x)Ge_(x))O_(y) wherein 0≦x≦1 and1.7≦y≦2.2, and

a metal having a work function of at least 4.5 eV or an oxide thereof.

11. The organic electroluminescent device of (7) wherein said metalhaving a work function of at least 4.5 eV is at least one memberselected from the group consisting of Au, Cu, Fe, Ni, Ru, Sn, Cr, Ir,Nb, Pt W, Mo, Ta, Pd, and Co.

12. The organic electroluminescent device of (10) wherein the content ofsaid metal and/or metal oxide is 0.2 to 40 mol %.

13. The organic electroluminescent device of (7) wherein said highresistance inorganic hole injecting layer has a thickness of 1 to 100nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of an organic EL device according to a first embodiment ofthe invention.

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of an organic EL device according to a second embodimentof the invention.

FIG. 3 is a schematic cross-sectional view illustrating a prior artorganic EL device.

FIG. 4 is a schematic cross-sectional view illustrating another priorart organic EL device.

DESCRIPTION OF PREFERRED EMBODIMENTS

The organic EL device of the invention has a hole injecting electrode,an electron injecting electrode, and at least one organic layer betweenthe electrodes, at least one of the organic layer having a lightemitting function. The device further has a high resistance inorganicelectron injecting layer between the electron injecting electrode andthe light emitting layer. The high resistance inorganic electroninjecting layer contains as a first component at least one metal oxidehaving a band gap of up to 4 eV selected from alkali metal elements,alkaline earth metal elements and lanthanide elements and as a secondcomponent at least one metal selected from among Sn Ru, V, Zn, Sm, andIn. The high resistance inorganic electron injecting layer is capable ofblocking holes and has conduction paths for carrying electrons.

By providing the inorganic electron injecting layer having electronconduction paths and capable of blocking holes between the electroninjecting electrode (or cathode) and the organic layer, it becomespossible to effectively inject electrons into the light emitting layerwhereby the luminous efficiency is improved and the drive voltage isreduced.

Also in the preferred inorganic electron injecting layer, the secondcomponent is contained in an amount of 0.2 to 40 mol % based on theentire components to form conduction paths. This enables effectiveinjection of electrons from the electron injecting electrode to theorganic layer on the light emitting layer side. Additionally, themigration of holes from the organic layer to the electron injectingelectrode is restrained, ensuring effective recombination of holes andelectrons in the light emitting layer. The organic EL device of theinvention has both the advantages of inorganic material and theadvantages of organic material. The organic EL device of the inventionproduces a luminance equal to that of prior art devices having anorganic electron injecting layer. Owing to high heat resistance andweather resistance, the organic EL device of the invention has a longerservice life than the prior art devices and develops minimal leaks anddark spots. Since not only a relatively expensive organic material, butalso an inexpensive, readily available, easy-to-prepare inorganicmaterial are used, the cost of manufacture can be reduced.

Preferably the high resistance inorganic electron injecting layer has aresistivity of 1 to 1×10¹¹ Ω-cm and especially 1×10³ to 1×10⁸ Ω-cm. Bycontrolling the resistivity of the inorganic electron injecting layerwithin this range, the efficiency of electron injection can bedrastically increased while maintaining high hole blockage. Theresistivity of the inorganic electron injecting layer may be determinedfrom a sheet resistance and a film thickness.

The high resistance inorganic electron injecting layer preferablycontains as the first component an oxide of any one of:

at least one alkali metal element selected from among Li, Na, K, Rb, Cs,and Fr,

at least one alkaline earth metal element selected from among Mg, Ca,and Sr, and

at least one lanthanide element selected from La and Ce, all the oxideshaving a work function of up to 4 eV. Of these, lithium oxide, magnesiumoxide, calcium oxide, and cerium oxide are preferable. When these oxidesare used in admixture, the mixture may have an arbitrary mix ratio.Further preferably, the mixture contains at least 50 mol % of lithiumoxide calculated as Li₂O.

The high resistance inorganic electron injecting layer further containsas the second component at least one element selected from among Zn, Sn,V, Ru, Sm, and In. The content of the second component is preferably 0.2to 40 mol %, more preferably 1 to 20 mol %. Outside the range, lowercontents would lead to a lower electron injecting function and highercontents would lead to a lower hole blocking function. When two or moreelements are used, the total content should preferably fall in the aboverange.

The oxides as the first component are generally present instoichiometric composition, but may deviate more or less therefrom andtake non-stoichiometric compositions. The second component is alsogenerally present as an oxide, and the same applies to the oxide of thesecond component.

The high resistance inorganic electron injecting layer may additionallycontain as impurities hydrogen and neon, argon, krypton, xenon and otherelements which are used as the sputtering gas, in a total amount of upto 5 at %.

As long as the overall inorganic electron injecting layer has theabove-described composition on the average, the layer need not beuniform in composition and may be of a structure having a gradedconcentration in a thickness direction.

The high resistance inorganic electron injecting layer is normallyamorphous.

The thickness of the high resistance inorganic electron injecting layeris preferably about 0.3 to 30 nm, especially about 1 to 20 nm. Theelectron injecting layer would fail to fully exert its function when itsthickness is outside the range.

Methods for preparing the inorganic electron injecting layer includevarious physical and chemical thin film forming methods such assputtering and evaporation, with the sputtering being preferred. Interalia, a multi-source sputtering method of separately sputtering targetsof the first and second components is preferable. The multi-sourcesputtering method permits appropriate sputtering means to be employedfor the respective targets. In the case of a single-source sputteringmethod, a target of a mixture of the first and second components may beused.

When the high resistance inorganic electron injecting layer is formed bysputtering, the sputtering gas is preferably under a pressure of 0.1 to1 Pa during sputtering. The sputtering gas may be any of inert gasesused in conventional sputtering equipment, for example, Ar, Ne, Xe, andKr. Nitrogen (N₂) gas may be used if necessary. Reactive sputtering maybe carried out in an atmosphere of the sputtering gas mixed with about 1to about 99% of oxygen (O₂) gas.

The sputtering process may be an RF sputtering process using an RF powersource or a DC sputtering process. The power of the sputtering equipmentis preferably in the range of 0.1 to 10 W/cm² for RF sputtering. Thedeposition rate is in the range of 0.5 to 10 nm/min., preferably 1 to 5nm/min.

The temperature of the substrate during deposition is from roomtemperature (25° C.) to about 150° C.

As shown in FIG. 1, for example, the organic EL device of the inventionmay have the successively stacked configuration of substrate 1/holeinjecting electrode 2/hole injecting and transporting layer 4/lightemitting layer 5/high resistance inorganic electron injecting layer6/electron injecting electrode 3. As opposed to the normally stackedconfiguration, the device may have the inversely stacked configurationof substrate 1/electron injecting electrode 3/high resistance electroninjecting layer 6/light emitting layer 5/hole injecting and transportinglayer 4/hole injecting electrode 2, as shown in FIG. 2. The inverselystacked configuration helps light emerge from the side of the assemblyopposite to the substrate. However, when the high resistance electroninjecting layer is deposited, the organic layer or the like can besubjected to ashing and hence, damaged. It is thus recommended that theelectron injecting layer is initially thinly deposited in the absence ofoxygen and then thickly in the presence of oxygen. The thickness reachedin the absence of oxygen is preferably about ⅕ to about ½ of the overallthickness. In FIGS. 1 and 2, a drive power supply E is connected betweenthe hole injecting electrode 2 and the electron injecting electrode 3.It is understood that the light emitting layer 5 is a light emittinglayer of broader definition including an electron injecting andtransporting layer, a light emitting layer of narrower definition, ahole transporting layer, and so on.

The device of the invention may have a multi-stage configuration ofelectrode layer/inorganic layer and light emitting layer/electrodelayer/inorganic layer and light emitting layer/electrode layer/inorganiclayer and light emitting layer/electrode layer, or further repeatedlayers. Such a multi-stage configuration is effective for adjusting ormultiplying the color of emitted light.

When combined with the aforementioned high resistance inorganic electroninjection layer, the electron injecting electrode (or negativeelectrode) need not have a low work function and electron injectingability and need not be specifically limited. Common metals may be used.Among others, one or more metal elements selected from among Al, Ag, In,Ti, Cu, Au, Mo, W, Pt, Pd, and Ni, especially Al and Ag are preferredfrom the standpoints of conductivity and ease of handling.

The negative electrode thin film may have at least a sufficientthickness to supply electrons to the inorganic electron injecting andtransporting layer, for example, a thickness of at least 50 nm,preferably at least 100 nm. Although the upper limit is not critical,the electrode thickness may typically be about 50 to about 500 nm.

As the electron injecting electrode, any of the following materials maybe used as needed. Exemplary materials include metal elements such as K,Li, Na, Mg, La, Ce, Ca, Sr, Ba, Sn, Zn, and Zr, and binary or ternaryalloys containing such metal elements for stability improvement, forexample, Ag—Mg (Ag: 0.1 to 50 at %), Al—Li (Li: 0.01 to 14 at %), In—Mg(Mg: 50 to 80 at %), and Al—Ca (Ca: 0.01 to 20 at %).

The electron injecting electrode thin film may have at least asufficient thickness to effect electron injection, for example, athickness of at least 0.1 nm, preferably at least 0.5 nm, morepreferably at least 1 nm. Although the upper limit is not critical, theelectrode thickness is typically about 1 to about 500 nm. On theelectron injecting electrode, an auxiliary or protective electrode maybe provided.

The auxiliary electrode may have at least a sufficient thickness toensure efficient electron injection and prevent the ingress of moisture,oxygen and organic solvents, for example, a thickness of at least 50 nm,preferably at least 100 nm, more preferably 100 to 500 nm. A too thinauxiliary electrode layer would exert its effect little, lose a stepcoverage capability, and provide insufficient connection to a terminalelectrode. If too thick, greater stresses are generated in the auxiliaryelectrode layer, accelerating the growth rate of dark spots.

For the auxiliary electrode, an appropriate material may be chosen inconsideration of the material of the electron injecting electrode to becombined therewith. For example, low resistivity metals such as aluminummay be used when electron injection efficiency is of importance. Metalcompounds such as TiN may be used when sealing is of importance.

The thickness of the electron injecting electrode and the auxiliaryelectrode combined is usually about 50 to about 500 nm though it is notcritical.

For the hole injecting electrode, materials capable of effectivelyinjecting holes into the hole injecting layer are preferred, with thosematerials having a work function of 4.5 to 5.5 eV being especiallypreferred. Useful are compositions based on tin-doped indium oxide(ITO), zinc-doped indium oxide (IZO), indium oxide (In₂O₃), tin oxide(SnO₂) or zinc oxide (ZnO). These oxides may deviate more or less fromtheir stoichiometric compositions. An appropriate proportion of SnO₂mixed with In₂O₃ is about 1 to 20%, more preferably about 5 to 12% byweight. For IZO, an appropriate proportion of ZnO mixed with In₂O₃ isabout 12 to 32% by weight.

The hole injecting electrode may further contain silicon oxide (SiO₂)for adjusting the work function. The content of silicon oxide (SiO₂) ispreferably about 0.5 to 10% as expressed in mol percent of SiO₂ based onITO. The work function of ITO is increased by incorporating SiO₂.

The electrode on the light exit side should preferably have a lighttransmittance of at least 50%, more preferably at least 60%, furtherpreferably at least 80%, especially at least 90% in the light emissionband, typically from 400 to 700 nm, and especially at each lightemission. With a lower transmittance, the light emitted by the lightemitting layer is attenuated through the electrode, failing to provide aluminance necessary as a light emitting device. It is noted that thelight transmittance of the electrode is sometimes set low for thepurpose of increasing the contrast ratio for improving visualperception.

Preferably the electrode has a thickness of 50 to 500 nm, especially 50to 300 nm. Although the upper limit of the electrode thickness is notcritical, a too thick electrode would cause a drop of transmittance andseparation. Too thin an electrode is insufficient for its effect and lowin film strength during fabrication.

In addition to the aforementioned high resistance inorganic electroninjecting layer, the organic EL device of the invention may have a highresistance hole injecting layer.

That is, the device further has a high resistance inorganic holeinjecting and transporting layer capable of blocking electrons andhaving conduction paths for carrying holes, between the hole injectingelectrode and the organic layer.

By providing the inorganic hole injecting layer having hole conductionpaths and capable of blocking electrons between the hole injectingelectrode and the organic layer, it becomes possible to effectivelyinject holes into the light emitting layer whereby the luminousefficiency is further improved and the drive voltage is reduced.

Also in the preferred inorganic insulative hole injecting layer, anoxide of silicon or germanium is used as the main component, and atleast one member selected from among metals and oxides, carbides,nitrides, and borides thereof having a work function of at least 4.5 eV,preferably 4.5 to 6 eV is contained to form conduction paths. Thisenables effective injection of holes from the hole injecting electrodeto the organic layer on the light emitting layer side. Additionally, themigration of electrons from the organic layer to the hole injectingelectrode is restrained, ensuring effective recombination of holes andelectrons in the light emitting layer. The organic EL device of theinvention has both the advantages of inorganic material and theadvantages of organic material. The organic EL device of the inventionproduces a luminance equal to that of prior art devices having anorganic hole injecting layer. Owing to high heat resistance and weatherresistance, the organic EL device of the invention has a longer servicelife than the prior art devices and develops minimal leaks and darkspots. Since not only a relatively expensive organic material, but alsoan inexpensive, readily available, easy-to-prepare inorganic materialare used, the cost of manufacture can be reduced.

Preferably the high resistance inorganic hole injecting layer has aresistivity of 1 to 1×10¹¹ Ω-cm and especially 1×10³ to 1×10⁸ Ω-cm. Bycontrolling the resistivity of the inorganic hole injecting layer withinthis range, the efficiency of hole injection can be drasticallyincreased while maintaining high electron blockage. The resistivity ofthe inorganic hole injecting layer may be determined from a sheetresistance and a film thickness. The sheet resistance may be measured bythe four-terminal method, for example.

The high resistance inorganic hole injecting layer preferably containsthe following insulative inorganic material and optionally, at least onemember selected from the group consisting of metals and oxides,carbides, nitrides, and borides thereof.

The inorganic insulative material is an oxide of silicon or germanium,preferably an oxide represented by the formula:

(Si_(1-x)Ge_(x)) O_(y)

wherein 0≦x≦1 and 1.7≦y≦2.2, especially 1.7≦y≦1.99. The inorganicinsulative hole injecting layer may be a thin film of silicon oxide orgermanium oxide or a mixture of silicon oxide and germanium oxide. If yis outside this range, the layer tends to reduce its hole injectingfunction. The composition may be examined by chemical analysis.

Preferably the high resistance inorganic hole injecting layer furthercontains a metal having a work function of at least 4.5 eV or an oxidethereof. The metal having a work function of at least 4.5 eV is one ormore of Au, Cu, Fe, Ni, Ru, Sn, Cr, Ir, Nb, Pt, W, Mo, Ta, Pd, and Co.These metals may also take the form of oxides, carbides, nitridessilicides or borides. When the metals are used in admixture, the mixturemay have an arbitrary mix ratio. The content of the metal is preferably0.2 to 40 mol %, more preferably 1 to 20 mol %. Outside the range, lowercontents would lead to a lower hole injecting function and highercontents would lead to a lower electron blocking function. When two ormore metals are used, the total content should preferably fall in theabove range.

The high resistance inorganic hole injecting layer may additionallycontain as impurities hydrogen and neon, argon, krypton, xenon and otherelements which are used as the sputtering gas, in a total amount of upto 5 at %.

As long as the overall inorganic hole injecting layer has theabove-described composition on the average, the layer need not beuniform in composition and may be of a structure having a gradedconcentration in a thickness direction.

The high resistance inorganic hole injecting layer is normallyamorphous.

The thickness of the high resistance inorganic hole injecting layer ispreferably about 1 to 100 nm, especially about 5 to 30 nm. The holeinjecting layer would fail to fully exert its function when itsthickness is outside the range.

Methods for preparing the inorganic hole injecting layer include variousphysical and chemical thin film forming methods such as sputtering andevaporation, with the sputtering being preferred. Inter alia, amulti-source sputtering method of separately sputtering a target of themain component and a target of the metal or metal oxide is preferable.The multi-source sputtering method permits appropriate sputtering meansto be employed for the respective targets. In the case of asingle-source sputtering method, the composition may be controlled byresting small pieces of the metal or metal oxide on a target of the maincomponent and properly adjusting the ratio of their areas.

The remaining sputtering conditions are the same as described above forthe high resistance electron injecting layer.

The light emitting layer is a thin film of an organic compoundparticipating in at least light emission or a multilayer film of two ormore organic compounds participating in at least light emission.

The light emitting layer has the functions of injecting holes andelectrons, transporting them, and recombining holes and electrons tocreate excitons. It is preferred that relatively electronically neutralcompounds be used in the light emitting layer so that electron and holesmay be readily injected and transported in a well-balanced manner.

The organic electron injecting and transporting layer has the functionsof facilitating injection of electrons from the electron injectingelectrode, transporting electrons stably, and obstructing holes. Thislayer is effective for increasing the number of holes and electronsinjected into the light emitting layer and confining holes and electronstherein for optimizing the recombination region to improve luminousefficiency.

The thicknesses of the light emitting layer and the electron injectingand transporting layer are not critical and vary with a particularformation technique although their thickness is usually preferred torange from about 5 nm to about 500 nm, especially about 10 nm to about300 nm.

The thickness of the electron injecting and transporting layer is equalto or ranges from about 1/10 times to about 10 times the thickness ofthe light emitting layer although it depends on the design of arecombination/light emitting region. When the electron injecting andtransporting layer is divided into an injecting layer and a transportinglayer, preferably the injecting layer is at least 1 nm thick and thetransporting layer is at least 1 nm thick. The upper limit of thicknessis usually about 500 nm for the injecting layer and about 500 nm for thetransporting layer. The same film thickness applies when twoinjecting/transporting layers are provided.

The light emitting layer of the organic EL device contains a fluorescentmaterial that is a compound having a light emitting function. Thefluorescent material may be at least one member selected from compoundsas disclosed, for example, in JP-A 264692/1988, such as quinacridone,rubrene, and styryl dyes. Also, quinoline derivatives such as metalcomplex dyes having 8-quinolinol or a derivative thereof as the ligandsuch as tris(8-quinolinolato)aluminum are included as well astetraphenylbutadiene, anthracene, perylene, coronene, and12-phthaloperinone derivatives. Further useful are the phenylanthracenederivatives described in JP-A 12600/1996 (Japanese Patent ApplicationNo. 110569/1994) and the tetraarylethene derivatives described in JP-A12969/1996 (Japanese Patent Application No. 114456/1994).

It is preferred to use such a compound in combination with a hostmaterial capable of light emission by itself, that is, to use thecompound as a dopant. In this embodiment, the content of the compound inthe light emitting layer is preferably 0.01 to 10% by weight, especially0.1 to 5% by weight. By using the compound in combination with the hostmaterial, the light emission wavelength of the host material can bealtered, allowing light emission to be shifted to a longer wavelengthand improving the luminous efficiency and stability of the device.

As the host material, quinolinolato complexes are preferable, withaluminum complexes having 8-quinolinol or a derivative thereof as theligand being more preferable. These aluminum complexes are disclosed inJP-A 264692/1988, 255190/1991, 70733/1993, 258859/1993 and 215874/1994.

Illustrative examples include tris(8-quinolinolato)aluminum,bis(8-quinolinolato)magnesium, bis(benzo{f}-8-quinolinolato)zinc,bis(2-methyl-8-quinolinolato)aluminum oxide,tris(8-quinolinolato)indium, tris(5-methyl-8-quinolinolato)aluminum,8-quinolinolatolithium, tris(5-chloro-8-quinolinolato)gallium,bis(5-chloro-8-quinolinolato)calcium,5,7-dichloro-8-quinolinolatoaluminum,tris(5,7-dibromo-8-hydroxyquinolinolato)aluminum, andpoly[zinc(II)-bis(8-hydroxy-5-quinolinyl)methane].

Also useful are aluminum complexes having another ligand in addition to8-quinolinol or a derivative thereof. Examples includebis(2-methyl-8-quinolinolato)(phenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(orthocresolato)aluminum(III),bis(2-methyl-8-quinolinolato)(metacresolato)aluminum(III),bis(2-methyl-8-quinolinolato)(paracresolato)aluminum(III),bis(2-methyl-8-quinolinolato)(ortho-phenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(para-phenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,3-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,6-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(3,4-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,6-diphenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,4,6-triphenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,3,6-trimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,3,5,6-tetramethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(1-naphtholato)aluminum(III),bis(2-methyl-8-quinolinolato)(2-naphtholato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(orthophenylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(para-phenylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum(III),bis(2-methyl-4-ethyl-8-quinolinolato)(para-cresolato)aluminum(III),bis(2-methyl-4-methoxy-8-quinolinolato)(paraphenylphenolato)aluminum(III),bis(2-methyl-5-cyano-8-quinolinolato)(ortho-cresolato)aluminum(III), andbis(2-methyl-6-trifluoromethyl-8-quinolinolato)(2-naphtholato)aluminum(III).

Also acceptable arebis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2,4-dimethyl-8-quinolinolato)aluminum(III),bis(4-ethyl-2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(4-ethyl-2-methyl-8-quinolinolato)aluminum(III),bis(2-methyl-4-methoxyquinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-4-methoxyquinolinolato)aluminum(III),bis(5-cyano-2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(5-cyano-2-methyl-8-quinolinolato)aluminum(III),andbis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum(III).

Other useful host materials are the phenylanthracene derivativesdescribed in JP-A 12600/1996 (Japanese Patent Application No.110569/1994) and the tetraarylethene derivatives described in JP-A12969/1996 (Japanese Patent Application No. 114456/1994).

The light emitting layer may also serve as the electron injecting andtransporting layer. In this case, tris(8quinolinolato)aluminum etc. arepreferably used. These fluorescent materials may be evaporated.

Also, if necessary, the light emitting layer may also. be a layer of amixture of at least one hole injecting and transporting compound and atleast one electron injecting and transporting compound, in which adopant is preferably contained. In such a mix layer, the content of thecompound is preferably 0.01 to 20% by weight, especially 0.1 to 15% byweight.

In the mix layer, carrier hopping conduction paths are created, allowingcarriers to move through a polarly predominant material while injectionof carriers of opposite polarity is rather inhibited, and the organiccompound becomes less susceptible to damage, resulting in the advantageof an extended device life. By incorporating the aforementioned dopantin such a mix layer, the light emission wavelength the mix layer itselfpossesses can be altered, allowing light emission to be shifted to alonger wavelength and improving the luminous intensity and stability ofthe device.

The hole injecting and transporting compound and electron injecting andtransporting compound used in the mix layer may be selected from thehole injecting and transporting compounds and the electron injecting andtransporting compounds to be described later, respectively. Inter alia,the hole injecting and transporting compound is preferably selected fromamine derivatives having strong fluorescence, for example,triphenyldiamine derivatives which are hole transporting materials,styrylamine derivatives and amine derivatives having an aromatic fusedring.

The electron injecting and transporting compound is preferably selectedfrom quinoline derivatives and metal complexes having 8-quinolinol or aderivative thereof as a ligand, especially tris(8-quinolinolato)aluminum(Alq3). The aforementioned phenylanthracene derivatives andtetraarylethene derivatives are also preferable.

As the hole injecting and transporting compound, amine derivativeshaving intense fluorescence are useful, for example, thetriphenyldiamine derivatives, styrylamine derivatives, and aminederivatives having an aromatic fused ring, exemplified above as the holeinjecting and transporting material.

The mix ratio is preferably determined in accordance with the carrierdensity and carrier mobility of the respective compounds. It is usuallypreferred that the weight ratio of the hole injecting and transportingcompound to the electron injecting and transporting compound range fromabout 1/99 to about 99/1, more preferably from about 10/90 to about90/10, especially from about 20/80 to about 80/20.

Also preferably, the thickness of the mix layer ranges from thethickness of a mono-molecular layer to less than the thickness of theorganic compound layer. Specifically, the mix layer is preferably 1 to85 nm, more preferably 5 to 60 nm, especially 5 to 50 nm thick.

Preferably the mix layer is formed by a co-deposition process ofevaporating the compounds from distinct sources. If both the compoundshave approximately equal or very close vapor pressures or evaporationtemperatures, they may be pre-mixed in a common evaporation boat, fromwhich they are evaporated together. The mix layer is preferably auniform mixture of both the compounds although the compounds can bepresent in island form. The light emitting layer is generally formed toa predetermined thickness by evaporating an organic fluorescent materialor coating a dispersion thereof in a resin binder.

In the electron injecting and transporting layer, there may be usedquinoline derivatives including organic metal complexes having8-quinolinol or a derivative thereof as a ligand such astris(8-quinolinolato)aluminum (Alq3), oxadiazole derivatives, perylenederivatives, pyridine derivatives, pyrimidine derivatives, quinoxalinederivatives, diphenylquinone derivatives, and nitro-substituted fluorenederivatives. The electron injecting and transporting layer can alsoserve as the light emitting layer. In this case, use oftris(8-quinolinolato)aluminum etc. is preferred. Like the light emittinglayer, the electron injecting and transporting layer may be formed byevaporation or the like.

Where the electron injecting and transporting layer is formed separatelyas an electron injecting layer and an electron transporting layer, twoor more compounds are selected in a proper combination from thecompounds commonly used in electron injecting and transporting layers.In this regard, it is preferred to stack layers in such an order that alayer of a compound having a greater electron affinity may be disposedadjacent the electron injecting electrode. The order of stacking alsoapplies where a plurality of electron injecting and transporting layersare provided.

In forming the organic hole injecting and transporting layer, the lightemitting layer, and the organic electron injecting and transportinglayer, vacuum evaporation is preferably used because homogeneous thinfilms are available. By utilizing vacuum evaporation, there is obtaineda homogeneous thin film which is amorphous or has a crystal grain sizeof less than 0.2 μm. If the grain size is more than 0.2 μm, uneven lightemission would take place and the drive voltage of the device must beincreased with a substantial drop of hole injection efficiency.

The conditions for vacuum evaporation are not critical although a vacuumof 10⁻⁴ Pa or lower and a deposition rate of about 0.01 to 1 nm/sec. arepreferred. It is preferred to successively form layers in vacuum becausethe successive formation in vacuum can avoid adsorption of impurities onthe interface between the layers, thus ensuring better performance.Also, the drive voltage of a device can be reduced and the developmentand growth of dark spots be restrained.

In the embodiment wherein the respective layers are formed by vacuumevaporation, where it is desired for a single layer to contain two ormore compounds, boats having the compounds received therein areindividually temperature controlled to achieve co-deposition.

Further preferably, a shield plate may be provided on the device inorder to prevent the organic layers and electrodes from oxidation. Inorder to prevent the ingress of moisture, the shield plate is attachedto the substrate through an adhesive resin layer for sealing. Thesealing gas is preferably an inert gas such as argon, helium, andnitrogen. The sealing gas should preferably have a moisture content ofless than 100 ppm, more preferably less than 10 ppm, especially lessthan 1 ppm. The lower limit of the moisture content is usually about 0.1ppm though not critical.

The shield plate is selected from plates of transparent or translucentmaterials such as glass, quartz and resins, with glass being especiallypreferred. Alkali glass is preferred because of economy although otherglass compositions such as soda lime glass, lead alkali glass,borosilicate glass, aluminosilicate glass, and silica glass are alsouseful. Of these, plates of soda glass without surface treatment areinexpensive and useful. Beside the glass plates, metal plates andplastic plates may also be used as the shield plate.

Using a spacer for height adjustment, the shield plate may be held at adesired height over the layer structure. The spacer may be formed fromresin beads, silica beads, glass beads, and glass fibers, with the glassbeads being especially preferred. Usually the spacer is formed fromparticles having a narrow particle size distribution while the shape ofparticles is not critical. Particles of any shape which does notobstruct the spacer function may be used. Preferred particles have anequivalent circle diameter of about 1 to 20 μm, more preferably about 1to 10 μm, most preferably about 2 to 8 μm. Particles of such diametershould preferably have a length of less than about 100 μm. The lowerlimit of length is not critical although it is usually equal to or morethan the diameter.

When a shield plate having a recess is used, the spacer may be used ornot. When used, the spacer should preferably have a diameter in theabove-described range, especially 2 to 8 μm.

The spacer may be premixed in a sealing adhesive or mixed with a sealingadhesion at the time of bonding. The content of the spacer in thesealing adhesive is preferably 0.01 to 30% by weight, more preferably0.1 to 5% by weight.

Any of adhesives which can maintain stable bond strength and gastightness may be used although UV curable epoxy resin adhesives ofcation curing type are preferred.

In the organic EL structure of the invention, the substrate may beselected from amorphous substrates of glass and quartz and crystallinesubstrates of Si, GaAs, ZnSe, ZnS, GaP, and InP, for example. Ifdesired, buffer layers of crystalline materials, amorphous materials ormetals may be formed on such crystalline substrates. Metal substratesincluding Mo, Al, Pt, Ir, Au and Pd are also useful. Of these, glasssubstrates are preferred. Since the substrate is often situated on thelight exit side, the substrate should preferably have a lighttransmittance as described above for the electrode.

A plurality of inventive devices may be arrayed on a plane. A colordisplay is obtained when the respective devices of a planar array differin emission color.

The substrate may be provided with a color filter film, a fluorescentmaterial-containing color conversion film or a dielectric reflectingfilm for controlling the color of light emission.

The color filter film used herein may be a color filter as used inliquid crystal displays and the like. The properties of a color filtermay be adjusted in accordance with the light emission of the organic ELdevice so as to optimize the extraction efficiency and color purity.

It is also preferred to use a color filter capable of cutting externallight of short wavelength which is otherwise absorbed by the EL devicematerials and fluorescence conversion layer, because the lightresistance and display contrast of the device are improved.

An optical thin film such as a multilayer dielectric film may be usedinstead of the color filter.

The fluorescence conversion filter film is to convert the color of lightemission by absorbing electroluminescence and allowing the fluorescentmaterial in the film to emit light. It is formed from three components:a binder, a fluorescent material, and a light absorbing material.

The fluorescent material used may basically have a high fluorescentquantum yield and desirably exhibits strong absorption in theelectroluminescent wavelength region. In practice, laser dyes areappropriate. Use may be made of rhodamine compounds, perylene compounds,cyanine compounds, phthalocyanine compounds (includingsub-phthalocyanines), naphthalimide compounds, fused ring hydrocarboncompounds, fused heterocyclic compounds, styryl compounds, and coumarincompounds.

The binder is selected from materials which do not cause extinction offluorescence, preferably those materials which can be finely patternedby photolithography or printing technique. Also, where the filter filmis formed on the substrate so as to be contiguous to the hole injectingelectrode, those materials which are not damaged during deposition ofthe hole injecting electrode (such as ITO or IZO) are preferable.

The light absorbing material is used when the light absorption of thefluorescent material is short and may be omitted if unnecessary. Thelight absorbing material may also be selected from materials which donot cause extinction of fluorescence of the fluorescent material.

The organic EL device of the invention is generally of the dc or pulsedrive type. The applied voltage is generally about 2 to 30 volts.

In addition to the display application, the organic EL device of theinvention may find use as various optical devices such as opticalpickups for use in reading and writing in storages, repeaters intransmission lines for optical communication, and photo couplers.

EXAMPLE

Examples of the invention are given below by way of illustration.

Example 1

A substrate of (7059) glass by Corning Glass Works was scrubbed using aneutral detergent.

By RF magnetron sputtering from a target of ITO oxide, a hole injectingelectrode layer of ITO having a thickness of 200 nm was formed on thesubstrate at a temperature of 250° C.

After its ITO electrode-bearing surface was cleaned with UV/O₃, thesubstrate was secured by a holder in a vacuum deposition chamber, whichwas evacuated to a vacuum of 1×10⁻⁴ Pa or lower.

With the vacuum kept,N,N,N′,N′-tetrakis(m-biphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) asevaporated at an deposition rate of 0.2 nm/sec to a thickness of 200 nm,forming a hole injecting and transporting layer.

With the vacuum kept,N,N,N′,N′-tetrakis(m-biphenyl)-1,1′-biphenyl-4,4′-diamine (TPD),tris(8-quinolinolato)aluminum (Alq3), and rubrene were evaporated at anoverall deposition rate of 0.2 nm/sec to a thickness of 100 nm, forminga light emitting layer. The layer consisted of a mixture of TPD and Alq3in a volume ratio of 1:1 doped with 10 vol % of rubrene.

Next, the substrate was transferred to a sputtering apparatus. Using atarget of Li₂O mixed with 4 mol % of V, a high resistance inorganicelectron injecting layer was deposited to a thickness of 10 nm. Thesputtering gas used was a mixture of 30 sccm of Ar and 5 sccm of O₂.Sputtering conditions included room temperature (25° C.), a depositionrate of 1 nm/min, an operating pressure of 0.2 to 2 Pa, and an inputpower of 500 W. The inorganic electron injecting layer as deposited hadsubstantially the same composition as the target.

Next, with the vacuum kept, Al was evaporated to a thickness of 100 nmto form a negative electrode. Final sealing of a glass shield completedan organic EL device.

The thus obtained organic EL device was driven in air at a constantcurrent density of 10 mA/cm², finding an initial luminance of 800 cd/m²and a drive voltage of 7.5 volts.

The sheet resistance of the high resistance inorganic electron injectinglayer was measured by the four-terminal method. The layer having athickness of 100 nm showed a sheet resistance of 10 kΩ/cm², whichcorresponds to a resistivity of 1×10⁹ Ω-cm.

Example 2

In Example 1, Li₂O was replaced by an oxide of at least one elementselected from the group consisting of alkali metal elements: Na, K, Rb,Cs, and Fr, alkaline earth metal elements: Be, Mg, Ca, Sr, Ba, and Ra,and lanthanide elements: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, and Lu, with equivalent results.

Similar results were obtained when V was replaced by at least oneelement selected from among Ru, Zn, Sm, and In.

Example 3

In the step of depositing the hole injecting and transporting layer inExamples 1 and 2, the substrate was transferred to the sputteringapparatus. Using a target of SiO₂ having a gold pellet of apredetermined size rested thereon, the high resistance inorganic holeinjecting layer was deposited to a thickness of 20 nm. The sputteringgas used was a mixture of 30 sccm of Ar and 5 sccm of O₂. Sputteringconditions included room temperature (25° C.), a deposition rate of 1nm/min, an operating pressure of 0.2 to 2 Pa, and an input power of 500W. The inorganic hole injecting layer as deposited had a composition ofSiO_(1.9) containing 4 mol % of Au.

Otherwise as in Example 1, an organic EL device was obtained. Theorganic EL device was driven at a constant current density of 10 mA/cm²as in Example 1, finding an initial luminance of 950 cd/m² and a drivevoltage of 7 volts.

Organic EL devices were fabricated as above except that in the step ofdepositing the high resistance inorganic hole injecting layer, the flowrate of O₂ in the sputtering gas was changed and the target used waschanged in accordance with the desired layer composition so that theresulting layers had the compositions SiO_(1.7), SiO_(1.95), GeO_(1.96),and Si_(0.5)Ge_(0.5)O_(1.92), respectively. The devices were tested foremission luminance, obtaining substantially equivalent results.

Comparative Example 1

The following changes were made when the high resistance inorganicelectron injecting layer was deposited in Example 1. With the vacuumkept, the substrate was transferred to the sputtering apparatus. Using atarget consisting of strontium oxide (SrO), lithium oxide (Li₂O), andsilicon oxide (SiO₂) mixed in the proportion:

SrO: 80 mol %,

Li₂O: 10 mol %,

SiO₂: 10 mol %,

based on the entire components, the inorganic electron injecting andtransporting layer was deposited to a thickness of 0.5 μm. Thedepositing conditions included a substrate temperature of 25° C.,sputtering gas Ar, a deposition rate of 1 nm/min, an operating pressureof 0.5 Pa, and an input power of 5 W/cm². While supplying Ar/O₂=1/1 at100 SCCM, the inorganic electron injecting and transporting layer wasdeposited.

Otherwise as in Example 1, an organic EL device was obtained. Theorganic EL device was driven in air at a constant current density of 10mA/cm², finding an initial luminance of 500 cd/m² and a drive voltage of10 volts.

When the inorganic electron injecting layer was changed to a thicknessof 10 rim, an initial luminance of 2 cd/in² and a drive voltage of 18volts were found at a constant current density of 10 mA/cm².

Comparative Example 2

An organic EL device was fabricated as in Example 1 except that the holeinjecting and transporting layer (Alq3) formed on the hole injectingelectrode in Example 1 was omitted.

The organic EL device was driven at a current density of 10 MA/cm²,finding an initial luminance of 850 cd/m² and a drive voltage of 7 V.

BENEFITS OF THE INVENTION

According to the invention, organic EL devices having an excellentelectron injection efficiency, improved luminous efficiency, lowoperating voltage, and low cost are realized.

What is claimed is:
 1. An organic electroluminescent device comprising ahole injecting electrode, an electron injecting electrode, at least oneorganic layer between the electrodes, at least one layer of said organiclayer having a light emitting function, and a high resistance inorganicelectron injecting layer between said electron injecting electrode andsaid light emitting layer, said high resistance inorganic electroninjecting layer comprising as a first component an oxide of at least oneelement selected from alkali metal elements, alkaline earth metalelements and lanthanide elements, having a work function of up to 4 eVand as a second component at least one metal having a work function of 3to 5 eV, and said high resistance inorganic electron injecting layerbeing capable of blocking holes and having conduction paths for carryingelectrons.
 2. The organic electroluminescent device of claim 1 whereinsaid second component is at least one metal selected from the groupconsisting of Zn, Sn, V, Ru, Sm, and In.
 3. The organicelectroluminescent device of claim 1 wherein said alkali metal elementsinclude Li, Na, K, Rb, Cs, and Fr, said alkaline earth metal elementsinclude Mg, Ca, and Sr, and said lanthanide element include La and Ce.4. The organic electroluminescent device of claim 1 wherein said highresistance inorganic electron injecting layer has a resistivity of 1 to1×10¹¹ Ω-cm.
 5. The organic electroluminescent device of claim 1 whereinsaid high resistance inorganic electron injecting layer contains 0.2 to40 mol % based on the entire components of the second component.
 6. Theorganic electroluminescent device of claim 1 wherein said highresistance inorganic electron injecting layer has a thickness of 0.3 to30 nm.
 7. The organic electroluminescent device of claim 1 furthercomprising a high resistance inorganic hole injecting layer between saidhole injecting electrode and said organic layer, said high resistanceinorganic hole injecting layer being capable of blocking electrons andhaving conduction paths for carrying holes.
 8. The organicelectroluminescent device of claim 7 wherein said high resistanceinorganic hole injecting layer has a resistivity of 1 to 1×10¹¹ Ω-cm. 9.The organic electroluminescent device of claim 7 wherein said highresistance inorganic hole injecting layer contains an insulative metalor metalloid and at least one member selected from the group consistingof oxides, carbides, nitrides, silicides and borides of metals.
 10. Theorganic electroluminescent device of claim 7 wherein said highresistance inorganic hole injecting layer contains silicon oxide orgermanium oxide or a mixture of silicon oxide and germanium oxide as amain component, the main component being represented by the formula:(Si_(1-x)Ge_(x))O_(y) wherein 0≦x≦1 and 1.7≦y≦2.2, and a metal having awork function of at least 4.5 eV or an oxide thereof.
 11. The organicelectroluminescent device of claim 7 wherein said metal has a workfunction of at least 4.5 eV and is at least one member selected from thegroup consisting of Au, Cu, Fe, Ni, Ru, Sn, Cr, Ir, Nb, Pt W, Mo, Ta,Pd, and Co.
 12. The organic electroluminescent device of claim 7 whereinthe content of said metal or metal oxide is 0.2 to 40 mol %.
 13. Theorganic electroluminescent device of claim 7 wherein said highresistance inorganic hole injecting layer has a thickness of 1 to 100nm.